In such systems, a need can arise to be able to operate with different pressures in the primary and secondary liquid circulation circuits, respectively, this normally being achieved by leading the primary liquid circulation circuit through a heat exchanger, and leading the secondary liquid circulation circuit through the heat exchanger separate from the primary circulation circuit by means of a pump. In addition to the possibility of operating with different pressures in the primary and secondary circuits, this arrangement also provides protection against liquid from the primary circulation circuit flowing out uncontrollably caused by a possible leak in the secondary circulatory circuit; this may be called for e.g. in district heating systems in order to protect against water damage. The heat exchanger will, however, introduce an undesired loss of heat, and will normally make it necessary to circulate the liquid in the secondary circulatory circuit by means of a circulation pump.